We were able to use size one when our baby was just below 5kg and have now been using these for 4 months. The fit works very well once you watch the instruction video on youtube. We love these and it is so rewarding to put a load of wash on that would otherwise have been a bag headed to landfill.
When researching cloth nappies before the birth of our baby these really seemed the best in terms of value for money. I think these are really easy to use as the bamboo insert dries quickly and it is easy to put them together again after drying. We used to have so many explosive poos and entire outfit changes with disposable nappies, but rarely have a leak with these.
I bought the fleecy liners to go with these nappies and would also highly recommend the bamboo wipes as they make cleaning pooey bums so much easier and you can just wash them along with the nappies.